1. Field of the Invention
The invention relates to a structure for shielding an electronic circuit from radio waves, and more particularly to a structure for shielding a radiofrequency circuit equipped with an antenna.
2. Description of the Related Art
A conventional structure for shielding a radio-frequency circuit is comprised of a shield case composed of metal for encasing a radio-frequency circuit therein to shield the circuit from radio-waves. Accordingly, since an antenna has to be arranged outside the shield case, the radio-frequency circuit is electrically connected to the antenna through a connector and a cable.
In order to solve this problem, Japanese Unexamined Utility Model Publication No. 64-40910 published on Mar. 10, 1989 has suggested such a plastic shield case as illustrated in FIGS. 1A and 1B. FIG. 1A is a perspective view of the suggested plastic shield case, and FIG. 1B is a cross-sectional view taken along the line 1B--1B in FIG. 1A.
The plastic shield case is constituted of an upper half 2a and a lower half 2b. A radio-frequency circuit (not illustrated) is formed on an upper surface of a printed wiring board 1. The upper and lower halves 2a and 2b are assembled in such a manner that the printed wiring board 1 is sandwiched between the upper and lower halves 2a and 2b. Each of the upper and lower halves 2a and 2b are formed with two cavities, and inner surfaces of those two cavities are covered with an electrically conductive layer 3 composed of metal for radio-waves shielding. The electrically conductive layer 3 partially extends outside the upper half 2a to thereby form an antenna element 4.
In accordance with the suggested shield case, the radio-frequency circuit can be electrically connected to the antenna element 4 without a connector and a cable.
However, the above-mentioned plastic shield case has to have the two halves 2a and 2b for sandwiching the printed wiring board 1 therebetween. The reason is as follows. If the printed wiring board 1 is shielded only at an upper surface on which an electronic circuit is mounted, external radio-waves enter the electronic circuit and internal radio-waves leaks from the electronic circuit both through a lower surface of the printed wiring board. Thus, it is absolutely necessary to shield the printed wiring board at upper and lower surfaces thereof
The use of the upper and lower halves 2a and 2b causes a problem that the shield structure is totally heavy, big in size, and high in fabrication cost.
In addition, in the above-mentioned plastic shield case, the antenna element 4 is formed on the upper half 2a separately from the printed wiring board 1 on which a radio-frequency circuit is formed. Hence, when the printed wiring board 1 is shielded with the upper and lower halves 2a and 2b, it is necessary to electrically connect the radio-frequency circuit to the antenna element 4.
As a result, assembly of the plastic shield case would take much time and much labor. In addition, when the plastic shield case is disassembled to take out the printed wiring board 1 for inspection and is assembled again, laborious work has to be done, specifically, the radio-frequency circuit is disconnected from the antenna element 4 and connected again to the antenna element 4, in which case, the radio-frequency circuit may be imperfectly, electrically connected to the antenna element 4.